An optical encoder has been known that includes a light source configured to emit parallel light, a plurality of diffraction gratings (scales) each having a grating face on which a plurality of grooves diffracting the parallel light are disposed in parallel, and a light-receiving unit (head) configured to receive the light diffracted at the diffraction gratings.
In this optical encoder, the parallel light emitted from the light source is diffracted into multiple diffracted light beams at the grooves of the diffraction gratings. The diffracted light beams generate an interference pattern having the same period as that of the diffraction gratings. The light-receiving unit detects a signal from the interference pattern. The optical encoder calculates the relative shift of the scale and the head from the result (signal) detected by the light-receiving unit.
The diffracted light beams include diffracted light traveling in the same direction as the optical axis of the light emitted from the light source, diffracted light traveling at predetermined diffraction angles on both sides of the optical axis, and diffracted light traveling at diffraction angles greater than the predetermined diffraction angles on both sides of the optical axis.
Among the diffracted light beams, the diffracted light beam traveling in the same direction as the optical axis is defined as a zeroth-order diffracted light beam. The other diffracted light beams are defined as ±first-order diffracted light beams and ±second-order diffracted light beams in the increasing order of the diffraction angle with reference to the zeroth-order diffracted light beam.
The light-receiving unit detects a signal primarily from an interference pattern generated from the ±first-order diffracted light. Consequently, the ±first-order diffracted light is defined as signal diffracted light, and the zeroth-order diffracted light and diffracted light of orders higher than the ±first-order diffracted light are defined as noise diffracted light.
When signal diffracted light and noise diffracted light are incident on the light-receiving unit, the interference pattern generated from the signal diffracted light is distorted by the noise diffracted light. As a result, the signal detected by the light-receiving unit contains noise. There is thus a problem in that relative shift calculated from the signal by the light-receiving unit has a low accuracy, and the optical encoder is less reliable.
In contrast, the interference-type position measurement device (optical encoder) described in Patent Document 1 includes an optical block and thereby removes noise diffracted light with the zeroth-order-diffracted-light shielding device and an integrated prism, to irradiate the scale with only the ±first-order diffracted light (signal diffracted light). However, such an optical encoder is provided with an integrated prism and/or a diffracted-light shielding device for blocking diffracted light excluding the ±first-order diffracted light. Hence, a space is required between the light source and the scale for reflection and refraction of the diffracted light beams. There is thus a problem in that the structure of the optical encoder becomes large.
FIG. 8 is a perspective view of a known optical encoder. FIGS. 9A and 9B illustrate an ideal interference pattern and an interference pattern containing noise diffracted light, respectively.
An averaging diffraction moire position detector (optical encoder) according to Patent Document 2 solves the problem of a size increase of the optical encoder through the following configuration.
With reference to FIG. 8, the averaging diffraction moire position detector includes a photomultiplier tube (light source) 300; a diffuser 400 having groove lines S; a first diffraction grating 401; a second diffraction grating 402 that is disposed parallel to the grading face of the first diffraction grating 401 and displaced in a direction orthogonal to the groove lines (grooves); and a unit 500 (light-receiving unit) that acquires detected results (signals) from the diffraction moire signals (interference pattern) derived from the two diffraction gratings 401 and 402 (a plurality of diffraction gratings).
The light beams from the photomultiplier tube 300 diffuse at the diffuser 400, pass through the first diffraction grating 401, and intersect each other at the second diffraction grating 402.
The two diffraction gratings 401 and 402 are disposed with a gap therebetween, the gap being created when disposing the first diffraction grating 401 and the second diffraction grating 402, such that the optical path length in the gap between the diffraction gratings 401 and 402 varies within the range of an optical path equivalent to integral multiple of the Fresnel number or two. Specifically, for example, in the case where the diffuser 400 and the two diffraction gratings 401 and 402 are disposed at equal intervals u along a freely-selected axis, the optical path length from the first diffraction grating 401 on one side of the freely-selected axis is u−Δu in a direction parallel to the grating face and orthogonal to the groove lines S. The optical path length on the other side of the freely-selected axis is u+Δu and is offset to the opposite direction. Hence, the second diffraction grating 402 tilts relative to the first diffraction grating 401.
The interference pattern generated on the light-receiving unit through the diffraction gratings is desirably a striped pattern, as illustrated in FIG. 9A. An interference pattern not including noise diffracted light and generated from only signal diffracted light is a striped pattern as illustrated in FIG. 9A. The light-receiving unit reads the interference pattern, for example, along the reading range C to detect a signal. The light-receiving unit can detect an ideal signal along any reading range even in a case where it is misaligned with the diffraction gratings because the interference pattern is a striped pattern.
In reality, however, the light passing through the diffraction gratings includes noise diffracted light, and thus, an interference pattern such as that illustrated in FIG. 9A cannot be generated.
In contrast, FIG. 9B illustrates an interference pattern from which an ideal signal can be detected even with noise. The interference pattern illustrated in FIG. 9B is generated, for example, in response to light from the photomultiplier tube 300 being diffused at the diffuser 400 and intersecting each other at the second diffraction grating 402 after passing through the first diffraction grating 401. The light-receiving unit 500 reads the interference pattern along, for example, a reading range C2 and conducts a predetermined calculation process on the read results, to acquire a signal similar to the ideal signal acquired through reading of the interference pattern illustrated in FIG. 9A. Although the reading range C2 needs to be wider than the reading range C for reading of the striped interference pattern illustrated in FIG. 9A, an ideal signal can be acquired from the interference pattern illustrated in FIG. 9B through a predetermined calculation process.
In the averaging diffraction moire position detector described in Patent Document 2, the second diffraction grating 402 tilts relative to the first diffraction grating 401. Thus, an interference pattern similar to that illustrated in FIG. 9B can be generated on the light-receiving unit. That is, an interference pattern similar to that from which an ideal signal can be acquired even with noise can be generated on the light-receiving unit.
Thus, the averaging diffraction moire position detector can generate an interference pattern similar to the interference pattern from which an ideal signal can be acquired without removal of noise with an integrated prism and/or a diffracted-light shielding device that blocks diffracted light other than the ±first-order diffracted light, by the second diffraction grating 402 tilted relative to the first diffraction grating 402. This prevents an increase in the size of the detector.